This invention relates to control valves for governing the operation of fluid motors, and it has more particular reference to valve instrumentalities by which the speed of motor operation can be closely controlled despite load influences which tend to drive the motor at rates faster than desired.
The tendency for fluid motors such as hydraulic cylinders to be driven by the load thereon is well known. The booms of backhoes and front end loaders, for example, constitute heavy loads which are customarily raised and lowered by hydraulic cylinders at the dictate of control valves therefor. During lowering of such a boom, the control valve for its cylinder directs pressure fluid from a pump into one end of the cylinder and conducts fluid expelled from the other end thereof to the reservoir line of the system. The boom, aided by gravity, tends to descend rapidly at an accelerating and uncontrolled rate, and the boom usually attains a speed such that the expanding end of its cylinder cannot be kept filled with fluid from the pump. When that occurs, a void is drawn in the expanding end of the cylinder, and positive control over the boom is lost until its cylinder is refilled with fluid.
In situations where the load is swingable from side to side, as is the case with the boom of a backhoe, the load can actually drive the cylinder in each direction. This makes it extremely difficult to control the speed with which the boom is swung, and positive control over the boom at all times cannot be had.
Elaborate and costly throttling schemes have been proposed in the past in an effort to achieve controlled movement of heavy loads by their hydraulic cylinders. This invention provides an exceptionally simple and low cost solution to that problem.